Pilot signal in an fdma communication system

ABSTRACT

Methods ( 500, 800 ) and corresponding systems ( 100, 200, 300, 400, 900 ) for generating a pilot symbol ( 330 ) include providing an M-point parallel transform sequence that is a discrete Fourier transform of a CAZAC sequence ( 312, 504 - 508 ). The M-point parallel transform sequence ( 312 ) is distributed ( 316, 510 ) to a set of M subcarriers among N subcarriers to form an N-point frequency-domain sequence ( 318 ) wherein the M subcarriers are evenly spaced apart. An N-point inverse fast Fourier transform ( 320, 512 ) is performed to convert the N-point frequency-domain sequence to an N-point time-domain sequence ( 322 ). The N-point time-domain sequence is converted ( 324, 514 ) to a serial sequence ( 326 ), and a cyclic prefix is added ( 328, 516 ) to the serial sequence to form a pilot symbol ( 330 ).

This application claims priority from and is a Divisional of applicationSer. No. 11/334,606 filed Jan. 18, 2006 entitled “Pilot Signal in anFDMA Communication system”.

FIELD OF THE INVENTION

This invention relates in general to communication systems using acarrier comprising multiple sub-carriers, and more specifically totechniques and apparatus for generating and using a pilot signal in amulti-carrier communication system.

BACKGROUND OF THE INVENTION

Multicarrier modulation systems divide the transmitted bitstream intomany different substreams and send these over many differentsubchannels. Typically the subchannels are orthogonal under idealpropagation conditions. The data rate on each of the subchannels is muchless than the total data rate, and the corresponding subchannelbandwidth is much less than the total system bandwidth. The number ofsubstreams is chosen to ensure that each subchannel has a bandwidth lessthan the coherence bandwidth of the channel, so the subchannelsexperience relatively flat fading. This makes the inter symbolinterference (ISI) on each subchannel small.

In more complex systems, which are commonly called orthogonal frequencydivision multiplexing (OFDM) systems (or multi-carrier or discretemulti-tone modulation systems), data is distributed over a large numberof carriers that are spaced apart at precise frequencies. The frequencyspacing provides the “orthogonality,” which prevents the demodulatorsfrom seeing frequencies other than their own. The benefits of OFDM arehigh spectral efficiency, resiliency to RF interference, and lowermulti-path distortion. This is useful because in a typical terrestrialbroadcasting scenario there are multipath-channels (i.e. the transmittedsignal arrives at the receiver using various paths of different length).Since multiple versions of the signal interfere with each other throughinter symbol interference (ISI), it becomes very hard for the receiverto extract the originally transmitted data.

In an OFDM system, data must be coherently demodulated. Therefore, it isnecessary to know the amplitude and phase of the channel in thereceiver. A pilot signal is transmitted with the data so that thereceiver can determine the amplitude and phase of the channel. The pilotsignal also allows the receiver to measure the transfer characteristicsof the channel between the transmitter and receiver through a processknown as “channel estimation.”

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, wherein like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which together with the detailed description below are incorporatedin and form part of the specification, serve to further illustratevarious embodiments and to explain various principles and advantages,all in accordance with the present invention.

FIG. 1 depicts, in a simplified and representative form, a high-levelblock diagram of portions of a single carrier frequency divisionmultiple access (SC-FDMA) transmitter for use in a data communicationssystem in accordance with one or more embodiments;

FIG. 2 shows, in a representative form, a high-level block diagram ofportions of an SC-FDMA receiver used to receive data transmitted by thetransmitter of FIG. 1 in accordance with one or more embodiments;

FIG. 3 depicts a more detailed high-level representative block diagramof portions of the SC-SDMA transmitter of FIG. 1 in accordance with oneor more embodiments;

FIG. 4 depicts a more detailed high-level representative block diagramof portions of the SC-SDMA receiver of FIG. 2 in accordance with one ormore embodiments;

FIG. 5 is a high-level flowchart of processes executed by the SC-SDMAtransmitter of FIGS. 1 and 3 in accordance with one or more embodiments;

FIGS. 6 and 7 show, in representative form, distributions of pilotsignal information to sets of subcarriers in accordance with one or moreembodiments;

FIG. 8 is a high-level flowchart of processes executed by the SC-SDMAreceiver of FIGS. 2 and 4 in accordance with one or more embodiments;and

FIG. 9 shows an alternative high-level representative block diagram ofportions of the SC-SDMA receiver of FIG. 2 in accordance with one ormore embodiments.

DETAILED DESCRIPTION

In overview, the present disclosure concerns a pilot signal to be usedfor estimating the transfer characteristics of a communication channelin a communication system. More particularly various inventive conceptsand principles embodied in methods and apparatus may be used forgenerating a pilot signal for transmitting with a data signal in acommunications system, and for demodulating the pilot signal in areceiver in the communication system.

While the pilot signal (generator/demodulator) of particular interestmay vary widely, one embodiment may advantageously be used in a wirelesscellular communications system having a transmitter and receiver using asingle carrier frequency division multiple access (SC-FDMA) modulationscheme. However, the inventive concepts and principles taught herein maybe advantageously applied to other broadband communications systemshaving multiplexed communication links transmitted in other media.

The instant disclosure is provided to further explain in an enablingfashion the best modes, at the time of the application, of making andusing various embodiments in accordance with the present invention. Thedisclosure is further offered to enhance an understanding andappreciation for the inventive principles and advantages thereof, ratherthan to limit in any manner the invention. The invention is definedsolely by the appended claims, including any amendments made during thependency of this application, and all equivalents of those claims asissued.

It is further understood that the use of relational terms, if any, suchas first and second, top and bottom, and the like, are used solely todistinguish one entity or action from another without necessarilyrequiring or implying any actual such relationship or order between suchentities or actions.

Much of the inventive functionality and many of the inventive principlesare best implemented with, or in, integrated circuits (ICs), includingpossibly application specific ICs, or ICs with integrated processingcontrolled by embedded software or firmware. It is expected that one ofordinary skill—notwithstanding possibly significant effort and manydesign choices motivated by, for example, available time, currenttechnology, and economic considerations—when guided by the concepts andprinciples disclosed herein will be readily capable of generating suchsoftware instructions and programs and ICs with minimal experimentation.Therefore, in the interest of brevity and minimization of any risk ofobscuring the principles and concepts according to the presentinvention, further discussion of such software and ICs, if any, will belimited to the essentials with respect to the principles and concepts ofthe various embodiments.

Referring to FIG. 1, a high-level diagram of portions of a singlecarrier frequency division multiple access (SC-FDMA) transmitter for usein a data communications system in accordance with one or moreembodiments will be briefly discussed and described. In FIG. 1, SC-FDMAtransmitter 100 includes data source 102, which generates a datasequence 104 that is traffic data or user data generated by, forexample, an application running in a subscriber unit of a cellularcommunications system, or perhaps by the transmission of streamingmedia, or by transferring a file, or other similar processes thattransfer data.

Because the transmitted signal will be coherently demodulated in areceiver, it is necessary to know the amplitude and phase of the channelin the receiver. A pilot signal is transmitted with the data so that thereceiver can determine the amplitude and phase of the channel. The pilotsignal also allows the receiver to measure the transfer characteristicsof the channel between the transmitter and receiver through a processknown as “channel estimation.”

As shown in FIG. 1, pilot sequence generator 106 generates a pilotsequence 108 that can be alternately and periodically transmitted withtraffic data 104 from data source 102. The pilot sequence 108 comprisesa known data sequence having known characteristics. The characteristicsof pilot sequence 108 are: (1) a constant magnitude; (2) zero circularautocorrelation; (3) a flat frequency domain response; and (4) acircular cross-correlation between two sequences that is low and has aconstant magnitude, provided that the sequence length is a prime number.

One sequence that has these properties is known as a Zadoff-Chu,Frank-Zadoff, and Milewski sequence, which is also known as aconstant-amplitude zero-autocorrelation (CAZAC) sequence. The CAZACsequence is defined as follows: Let L be any positive integer, and let kbe any number which is relatively prime with L. Then the n-th entry ofthe k-th Zadoff-Chu CAZAC sequence is given as follows:

$\begin{matrix}{{c_{k}(n)} = {\exp \left\lbrack {\frac{{j2\pi}\; k}{L}\left( {n + {n\frac{n + 1}{2}}} \right)} \right\rbrack}} & {{if}\mspace{14mu} L\mspace{14mu} {is}\mspace{14mu} {odd}} \\\begin{matrix}{{c_{k}(n)} = {\exp\left\lbrack {\frac{{j2\pi}\; k}{L}\left( {n + \frac{n^{2}}{2}} \right)} \right\rbrack}} & \;\end{matrix} & {{if}\mspace{14mu} L\mspace{14mu} {is}\mspace{14mu} {even}}\end{matrix}$

where n ranges from 0 to L−1.

The CAZAC sequence is a special subset of polyphase Chirp sequences,such as the Generalized Chirp Like (GCL) sequences. Note that the“constant-amplitude zero-autocorrelation” property of the CAZAC sequenceis preserved as the sequence is transformed to and from the time andfrequency domains. Thus, the CAZAC sequence has benefits for the radiofrequency power amplifier because transmission using SC-FDMA techniquesprovides a signal with a low peak-to-average power ratio in the timedomain. Additionally, the CAZAC sequence aids channel estimation becausethe signal has a constant amplitude in the frequency domain.

Switch controller 110 controls switch 112, which alternates betweenselecting data sequence 104 and pilot sequence 108. In one embodiment,switch controller 110 allows 6 symbols of data sequence 104 for every 2pilot sequences 108. Other ratios of data and pilot sequences may beused.

After being selected by a switch 112, pilot sequence 108 is coupled toserial-to-parallel converter 114. Serial-to-parallel converter 114receives a serial stream of data and converts it to a parallel dataoutput. If pilot sequence 108 is received, serial-to-parallel converter114 converts it to an M-point parallel CAZAC sequence 116, where M isthe number of complex valued samples in the sequence.

The M-point parallel output of serial-to-parallel converter 114 iscoupled to the input of single-carrier frequency division multipleaccess (SC-FDMA) modulator 118. SC-FDMA modulator 118 is a multi-carriermodulation block that uses digital mathematical techniques to generatethe orthogonal carriers required for OFDM transmission. The output ofSC-FDMA modulator 118 is a digital baseband representation of an OFDMmodulated pilot symbol 120. A more detailed view of the digitalprocessing system of data transmitter 100 is shown in FIG. 3, discussedbelow.

The OFDM modulated pilot symbol 120 is coupled to transmitter block 122.Within transmitter block 122, the digital signal is converted to ananalog signal by a digital-to-analog (D/A) converter. The analog signalis then coupled to an upconverter, and a power amplifier, to produceamplified radio frequency signal 124. Radio frequency signal 124 iscoupled to antenna 126 for transmitting transmitted radio signal 128.Transmitted radio signal 128 is affected by the environment according tothe characteristics of the channel, and thus signal 128 becomes a“received” signal at the receiver.

Referring to FIG. 2, there is depicted a representative block diagram ofa receiver that illustrates functional blocks for receiving,demodulating, and using a pilot signal in accordance with one or moreembodiments. As shown, receiver 200 includes antenna 202 for receivingreceived signal 204. Antenna 202 is coupled to downconverter andanalog-to-digital (A/D) converter 206, which removes the radio frequencycarrier from received signal 204 and converts the analog signal to aserial stream of digital samples 208.

Digital samples 208 from downconverter and A/D converter 206 are coupledto synchronization controller 210 and OFDM multi-carrier demodulator212. Synchronization controller 210 monitors digital samples 208 todetect a pilot signal using pilot signal detector 214. The purpose ofsynchronization controller 210 is to recover a reference signal (e.g., apilot signal), which is a known signal that provides symbol timing orsynchronization information, and which also allows channel estimationand the calculation of channel equalization coefficients. Suchsynchronization and channel estimation information 216 is coupled toOFDM multi-carrier demodulator 212 so that demodulator 212 may beprecisely synchronized in order to demodulate the OFDM signalrepresented by digital samples 208. OFDM multi-carrier demodulator 212provides, at its output, traffic data or user data 218, which is datacarried in received signal 204. Functions similar to those within OFDMmulti-carrier demodulator 212 may be used within synchronizationcontroller 210 to receive and demodulate pilot symbols.

With reference now to FIG. 3, there is depicted a more detailedrepresentative diagram of portions 300 of transmitter 100 (see FIG. 1)for generating a pilot symbol in accordance with one or moreembodiments. As illustrated in FIG. 3, a serial pilot sequence 304 isgenerated in short CAZAC sequence generator 302. In one embodiment, thesequence is M complex samples in length, wherein M is equal to, forexample, 37.

The CAZAC sequence generated by CAZAC sequence generator 302 is referredto as “short” sequence because the length of the sequence is shorterthan the number of available subcarriers. For example, if there are 301available subcarriers, a long CAZAC sequence would occupy all of them,and a short CAZAC sequence may only occupy, for example, 37 of them. Asnoted above, the “constant-amplitude zero-autocorrelation” property ofthe CAZAC sequence is preserved in both the time and frequency domains.

The output of short CAZAC sequence generator 302 is coupled toserial-to-parallel converter 306, which converts serial pilot sequence304 into an M-point parallel CAZAC sequence 308. M-point parallel CAZACsequence 308 is coupled to an input of M-point discrete Fouriertransformer (DFT) 310. M-point DFT 310 analyzes the frequency componentsof the M-points of serial pilot sequence 304, and converts serial pilotsequence 304 into the frequency domain to output an M-point paralleltransform sequence 312.

Note that in some embodiments, M-point parallel transform sequence 312may be recalled from transform memory 314. Thus, rather than generatingsequences and calculating discrete Fourier transforms (i.e. real-timesynthesis), the system and process may recall a pre-calculated M-pointparallel transform sequence 312 from a data storage device using a tablelook up technique.

M-point parallel transform sequence 312 is coupled to the input ofdistributed subcarrier mapper 316, which is used to distribute theM-point parallel transform sequence 312 to a set of M subcarriers amongN subcarriers to form an N-point frequency-domain sequence 318, where Nis greater than M and wherein the M subcarriers are evenly spaced apartamong the N subcarriers. N is the number of subcarriers used in theSC-FDMA transmitted signal. In one embodiment, N is equal to 512, and Nmaybe larger or smaller in alternative embodiments. Outputs 318 ofdistributed subcarrier mapper 316 that are not mapped to one of theM-point parallel transform sequence 312 values are set to a zero value.

Referring again to the output of distrubuted subcarrier mapper 316,N-point frequency-domain sequence. 318 is coupled. to N-point. inversefast Fourier transformer (IFFT) 320. N-point IFFT 320 converts N-pointfrequency-domain sequence 318 into N-point time-domain sequence 322. Theinverse fast Fourier transform is an efficient mathematical algorithmfor reversing a fast Fourier transform. N-point time-domain sequence 322is coupled to an input of parallel-to-serial converter 324, whichconverts the parallel data into serial sequence 326.

Serial sequence 326 is coupled to an input of cyclic prefix adder 328.Cyclic prefix adder 328 copies a predetermined number of complex samplesfrom the end of serial sequence 326 and places those samples at thebeginning of serial sequence 326. In one embodiment, the number ofsamples copied is 32. The purpose of adding a cyclic prefix is to ensureorthogonality, which prevents one subcarrier from interfering withanother (which is called intercarrier interference, or ICI). The outputof cyclic prefix headers 328 is pilot symbol 330.

Note that the functional blocks within dashed box 332 may be referred toas an SC-FDMA modulator, such as SC-FDMA modulator 118 in FIG, 1. Alsonote that in other embodiments, the order of functional blocksparallel-to-serial converter 324 and cyclic prefix adder 328 may bereversed so that the cyclic prefix is added before converting the datato a serial stream.

Turning now to FIG. 4, there is depicted a more detailed high-levelrepresentative block diagram of portions 400 of the SC-SDMA receiver 200of FIG. 2 in accordance with one or more embodiments. As illustrated,received waveform 402, which is a baseband digital stream similar todigital samples 208 in FIG. 2, is coupled to the input of cyclic prefixremover 404. Cyclic prefix remover 404 removes the cyclic prefix fromthe beginning of the digital samples to produce modified pilot waveform406.

Modified pilot waveform 406 is coupled to an input ofserial-two-parallel converter 408. The output of serial-to-parallelconverter 408 is N-point parallel modified pilot symbol 410, which iscoupled to an input of N-point fast Fourier transformer (FFT) 412.

N-point FFT 412 produces received transformed pilot symbol 414 at itsoutput, as it converts the pilot signal from the time domain to thefrequency domain.

Received transformed pilot symbol 414 is coupled to an input ofdistributed subcarrier de-mapper 416. Distributed subcarrier de-mapper416 de-maps the M distributed subcarriers in the received transformedpilot symbol to produce an M-point received signal 418. Distributedsubcarrier demapper 416 uses carrier mapping information 420 to performthe demapping function.

Carrier mapping information 420 describes the selected set of Msubcarriers (Le., the locations of the subcarriers containing pilotinformation within the N received subcarriers). Carrier mappinginformation 420 is known in the receiver before receiving receivedwaveform 402. Such carrier mapping information can be agreed uponaccording to a standard describing the data communication interface, orit can be transmitted in a control message the to the receiver before itis needed.

M-point received signal 418 is coupled to the input of M-point parallelmultiplier 422. M-point parallel multiplier 422 multiplies receivedsignal 418 by M-point channel estimate multiplier 424 to produce anM-point intermediate channel estimate 426. M-point channel estimatemultiplier or sequence 424 is, in one or more embodiments, a Fouriertransformed time-reversed conjugate sequence derived from serial pilotsequence 302 (see FIG. 3). The net effect of the transformation of, andmultiplication by, the pilot sequence is that the frequency domainequivalent of a time domain circular correlation with the pilot sequenceis performed

Following M-point parallel multiplier 422, M-point intermediate channelestimate 426 is coupled to two dimensional (2D) interpolator 428. Theoutput of 2D interpolator 428 is final channel estimate 430. The finalchannel estimate gives the channel estimate at the location of eachassigned data carrier.

Referring now to FIG. 5, there is depicted a high-level flowchart 500 ofexemplary processes executed by portions of a transmitter, such astransmitter 100, which is shown in the system of FIGS. 1 and 3, or othersimilar apparatus, in accordance with one or more embodiments. Asillustrated, the process begins at block 502, and thereafter passes toblock 504 wherein the process generates a short M-pointconstant-amplitude zero autocorrelation (CAZAC) serial pilot sequence,wherein M is an integer representing the number of complex samples inthe sequence. M is also referred to as a short sequence because M issignificantly less than a number of carriers used when mappingtransmitted data symbols on subcarriers of the SC-FDMA modulator. Thisis important because it allows the pilot symbols of different users tobe orthogonal in the frequency domain. Furthermore, it allows the sameCAZAC sequence to be reused by multiple users at different frequencyoffsets.

As shown in FIG. 3, this process is implemented by short CAZAC sequencegenerator 304, which generates pilot sequence 302. This process may beimplemented by recalling complex samples of sequence 302 from datamemory 314 (see FIG. 3). In other embodiments, short CAZAC sequence 302may be generated using specially designed logic circuits, or byexecuting specially programmed software code (e.g., microcode in amicrochip).

As discussed above, the CAZAC sequence has a constant magnitude, zerocircular autocorrelation, flat frequency-domain response, and lowcross-correlation between two sequences. In other embodiments, the pilotsequence may be implemented with a GCL sequence having similarcharacteristics.

The CAZAC pilot symbol has the properties of a CDMA signal in that it isable to average interference because of its superior correlationproperties. In addition, the signal has the benefits of frequency domainorthogonality. This means that the same CAZAC sequence can be reused ondifferent frequency sets of M subcarriers by different users. Thus, thesame CAZAC sequence can be used to estimate different user's channels.

Next, the process converts the M-point CAZAC pilot sequence to anM-point parallel pilot sequence, as illustrated at block 506. Thisprocess may be implemented by a serial-to-parallel converter, such asserial-to-parallel converter 114 in FIG. 1.

After converting the pilot sequence to parallel data, the processperforms an M-point discrete Fourier transform to produce an M-pointparallel transform sequence, as illustrated at block 508. This processmay be implemented in the M-point discrete Fourier transform block 304in FIG. 3. The M-point parallel transform sequence is a frequency-domainrepresentation of the time-domain modulation of the pilot sequence.

After transforming to the frequency-domain, the process distributes theM-point parallel transform sequence to a selected set of M subcarriersamong N subcarriers, as depicted at block 510. In this distributionprocess, N is the number of subcarriers transmitted in the OFDM signal,and N is greater than M. The M selected subcarriers are evenly spacedapart, as represented in FIGS. 6 and 7. In one embodiment, thisdistribution process is implemented in distributed subcarrier mapper 308shown in FIG. 3. Note that the N subcarriers that have not been selectedin the set of M subcarriers will have their values set to zero.

As shown in FIGS. 6 and 7, different sets of M subcarriers may beselected among the N subcarriers while still maintaining even carrierspacing in the group of N subcarriers. In FIG. 6 for example, a firstsubcarrier 602, labeled SC₀, is selected, and every fourth subcarrier604 is selected among a group of sixteen subcarriers 600, SC₀-SC₁₅.These selected subcarriers 602, 604 may represent a first group or setof selected subcarriers having a carrier spacing 606. Similarly, FIG. 7shows the selection of a second set of subcarriers 702, 704, beginningwith the second subcarrier 702, SC₁, and every fourth subcarrier 704thereafter. In FIG. 7, the carrier spacing 606 remains at four carriersapart. In practice, N will probably be much larger than sixteen. In oneembodiment, N is equal to 512, and M is equal to 37. Different sets ofsubcarriers may be selected in alternate frames of pilots symbols.Information specifying which set of selected subcarriers is used shouldbe known in advance in the receiver for proper demodulation. Suchinformation may be known by communicating the selected set in advancethrough a control message, or by agreeing upon a known sequence of setsbeginning at a selected time.

Next, after distribution, the process performs an N-point inverse fastFourier transform (IFFT) to convert the N-point frequency-domainsequence to an N-point time-domain sequence, as illustrated at block512. This process is implemented using N-point IFFT block 312 in FIG. 3.The IFFT converts the mapped set of M-points in the parallel transformsequence, and the zero values for the non-selected subcarriers, to anN-point distrubuted mode time-domain sequence.

Next, the process converts the N-point time-domain sequence to a serialsequence, as illustrated at block 514. Then, at block 516, the processadds a cyclic prefix to the serial sequence. The purpose of adding thecyclic prefixes to reduce inter symbol interference. This process isimplemented by copying a number of complex samples from the end of theserial sequence to the beginning of the serial sequence, as is known inthe art of OFDM modulation. In one embodiment, the number of complexsamples copied is equal to 32.

Finally, the process of generating a pilot symbol in an OFDM transmitterends at block 518. It should be apparent that while the process ofgenerating a pilot signal ends at block 518, the process depicted may berepeated as necessary to provide multiple pilot symbols as dictated bythe requirements of the particular system in which the process is used.

Referring now to FIG. 8, there is depicted a high-level flowchart 800 ofexemplary processes executed by portions of a receiver, such as receiver400, which is shown in the system of FIGS. 2 and 4, or other similarapparatus, in accordance with one or more embodiments. As illustrated,the process begins at block 802, and thereafter passes to block 804wherein the process receives the OFDM signal. This process of receivingthe OFDM signal includes downconverting the received signal andconverting the down converted analog waveform to digital samples. Asshown in FIG. 2, receiving the OFDM signal is implemented indownconverter and A/D converter 206, which produces received waveform402 shown in FIG. 4. Note that the received signal 204 is equal to thetransmitted signal 126 multiplied by the transfer function of thechannel.

Next, the process removes the cyclic prefix from the received pilotwaveform to produce a modified pilot waveform, as depicted at block 806.Removing the cyclic prefix is implemented in the cyclic prefix removerblock 404 shown in FIG. 4. After removing the cyclic prefix, the processconverts the serial modified pilot waveform into an N-point parallelmodified pilot symbol, as illustrated at block 808.

Next, the process performs an N-point fast Fourier transform on theN-point parallel modified pilot symbol to produce a received transformedpilot symbol, as depicted at block 810, This process converts a signalin the time-domain to a signal in the frequency-domain.

After the fast Fourier transform, the process de-maps M distrubutedsubcarriers in the received transformed pilot symbol to produce anM-point received signal as illustrated at block 812. This process may beimplemented using distrubuted subcarrier de-mapper 416 in FIG. 4.

Next, the process multiplies the M-Point received pilot symbol by anM-point channel estimate multiplier to produce an M-point intermediatechannel estimate, as depicted at block 814. The M-point channel estimatemultiplier is derived from the transmitted pilot symbol, and is aFourier transformed time-reversed conjugate sequence of the transmittedpilot symbol.

After multiplying, the process performs a two dimensional interpolationto produce a final channel estimate, as illustrated at block 816. In oneembodiment, the two dimensional interpolation is performed using 2Dinterpolator 428 in FIG. 4.

Finally, the process of receiving and using a pilot symbol in an OFDMreceiver ends at block 818. It should be apparent that while theprocessing of the pilot signal ends at block 818, the process depictedmay be repeated as necessary to receive and process multiple pilotsymbols as dictated by the requirements of the particular system inwhich the process is used.

With reference now to FIG. 9, there is depicted high-levelrepresentative block diagram of an alternate embodiment of portions 900of the SC-SDMA receiver 200 of FIG. 2 in accordance with one or moreembodiments. The functional blocks comprising receiver 900 are similarto those of receiver 200, except for the addition of the followingfunctional blocks: M-point inverse discrete Fourier transform (IDFT)902, interference mitigation 904, and M-point DFT 906. The purpose ofthe IDFT is to convert the pilot signal into the time domain whereadditional processing and optimization can be performed to mitigateinterference and multipath. This produces a refined time-domain channelestimate. The refined time-domain channel estimate must then betransformed back into the frequency domain for interpolation.

The above described functions and structures can be implemented in oneor more integrated circuits. For-example, many or all of the functionscan be implemented in the signal processing circuitry that is suggestedby the block diagrams shown in FIGS. 1-4 and 9.

The processes, apparatus, and systems, discussed above, and theinventive principles thereof are intended to produce an improved andmore efficient pilot symbol in an SC-FDMA transmitter and receiversystem by combining CDMA and FDMA pilot signals. By using a generalizedchirp like (GCL) sequence—such as the CAZAC sequence—for the pilotsignal, the peak to average ratio of the transmitted signal can belowered, and the characteristics of the channel may be estimated moreaccurately because the pilot signal has a constant amplitude in thefrequency-domain, which is better suited for channel estimation. When aCAZAC sequence is inserted in an FDMA manner for the pilot channel, thereceiver may use interference averaging techniques to receive the pilotsignal without sacrificing the link benefits of FDMA pilots Thesesignificant improvements can be made with relatively low cost andminimal added complexity.

While the embodiments discussed above primarily relate to transmitting aradio frequency signal in a wireless communications system, this systemfor generating a pilot symbol, and processes therein, may be used inother data transmission applications, such as transmitting data via awireline media, such as a wideband coaxial cable, twisted-pair telephonewire, or the like.

This disclosure is intended to explain how to fashion and use variousembodiments in accordance with the invention, rather than to limit thetrue, intended, and fair scope and spirit thereof. The foregoingdescription is not intended to be exhaustive or to limit the inventionto the precise form disclosed. Modifications or variations are possiblein light of the above teachings. The embodiment(s) were chosen anddescribed to provide the best illustration of the principles of theinvention and its practical application, and to enable one of ordinaryskill in the art to utilize the invention in various embodiments andwith various modifications as are suited to the particular usecontemplated. All such modifications and variations are within the scopeof the invention as determined by the appended claims, as may be amendedduring the pendency of this application for patent, and all equivalentsthereof, when interpreted in accordance with the breadth to which theyare fairly, legally, and equitably entitled.

1.-6. (canceled)
 7. A method for receiving a pilot symbol in an singlecarrier frequency division multiple access (SC-FDMA) receivercomprising: removing a cyclic prefix from a received sequence to producea modified sequence; transforming the modified sequence to a firstfrequency domain sequence according to a first transform; demapping aplurality of distributed subcarriers in the transformed modifiedsequence to extract a plurality of received pilot symbols; deriving anintermediate channel estimate for each of the plurality of receivedpilot symbols, based on one or more characteristics of each of theplurality of pilot symbols; and interpolating a final channel estimatebased on the plurality of derived intermediate channel estimates.
 8. Themethod of claim 7, wherein the first transform comprises a discreteFourier transform.
 9. The method of claim 7, wherein the deriving theintermediate channel estimate is based on at least one of the one ormore predefined characteristics of the received plurality of pilotsymbols.
 10. The method of claim 9, wherein the one or more predefinedcharacteristics comprises a constant magnitude property.
 11. The methodof claim 9, wherein the one or more predefined characteristics comprisesa zero circular autocorrelation property.
 12. The method of claim 9,wherein the one or more predefined characteristics comprises a flatfrequency domain response property.
 13. The method of claim 9, whereinthe one or more predefined characteristics comprises a low and constantmagnitude result when circularly cross-correlated with another sequence.14. The method of claim 7, wherein the plurality of received pilotsymbols comprises a Zadoff-Chu sequence.
 15. The method of claim 7,further comprising demapping a second plurality of subcarriers toextract a datastream.
 16. The method of claim 7, wherein the pluralityof distributed subcarriers in the transformed modified sequence aredistributed substantially evenly throughout the transformed modifiedsequence.
 17. The method of claim 7, wherein the distribution of thedistributed subcarriers in the transformed modified sequence changesbetween successive receptions of the received sequence.
 18. A singlecarrier frequency division multiple access (SC-FDMA) receiver,comprising: a wireless interface comprising: first apparatus configuredto remove a cyclic prefix from a received sequence to produce a modifiedsequence; second apparatus configured to transform a received sequenceto a first frequency domain sequence; third apparatus configured todemap a plurality of distributed subcarriers in the first frequencydomain sequence to extract a plurality of received pilot symbols; fourthapparatus configured to derive an intermediate channel estimate for eachof the plurality of received pilot symbols, based on one or morecharacteristics of each of the plurality of pilot symbols; and fifthapparatus configured to interpolate a final channel estimate based onthe plurality of derived intermediate channel estimates.
 19. The SC-FDMAreceiver of claim 18, wherein the second apparatus is configured toexecute a discrete Inverse Fourier Transform.
 20. The SC-FDMA receiverof claim 18, wherein the third apparatus is additionally configured todemap a second plurality of distributed subcarriers in the firstfrequency domain sequence to extract a plurality of received datasymbols;
 21. The SC-FDMA receiver of claim 20, wherein the cyclic prefixis present within the received sequence in order to ensureorthogonality.
 22. The SC-FDMA receiver of claim 18, additionallycomprising apparatus configured to perform serial to parallel conversionin communication with the first and second apparatus.
 23. The SC-FDMAreceiver of claim 17, wherein the plurality of distributed subcarriersis distributed substantially evenly throughout the first frequencydomain sequence.
 24. The SC-FDMA receiver of claim 23, wherein thedistribution of the plurality of distributed subcarriers changes betweensuccessive receptions of the received sequence.
 25. A method forextracting one or more pilot symbols within an Orthogonal FrequencyDivision Multiplexing (OFDM) system, comprising: receiving a sequencecomprising a plurality of pilot symbols and data symbols interspersed inthe frequency domain; extracting the plurality of pilot symbols and datasymbols from the received sequence; estimating an intermediate channelresponse for each of the extracted pilot symbols; and interpolating anoverall channel response based at least in part on the estimatedintermediate channel response.
 26. The method of claim 25, wherein thepilot symbols comprise a Zadoff-Chu sequence.
 27. The method of claim25, where the overall channel response is applied to the extracted datasymbols.
 28. Wireless receiver apparatus configured to extract one ormore pilot symbols from a wireless Orthogonal Frequency DivisionMultiplexing (OFDM) signal, comprising: first apparatus configured toreceive a sequence comprising a plurality of pilot symbols and datasymbols interspersed in the frequency domain; second apparatusconfigured to extract the plurality of pilot symbols and data symbolsfrom the received sequence; third apparatus configured to estimate anintermediate channel response for each of the extracted pilot symbols;and fourth apparatus configured to interpolate an overall channelresponse based at least in part on the estimated intermediate channelresponse.
 29. The receiver apparatus of claim 28, wherein the pilotsymbols comprise a Zadoff-Chu sequence,
 30. The receiver apparatus ofclaim 28, where the overall channel response is applied to the extracteddata symbols.